An MR sensor detects magnetic field signals through the resistance changes of a read element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the read element. The conventional MR sensor, such as that used in the IBM "Corsair" disk drive, operates on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine of the angle between the magnetization in the read element and the direction of sense current flow through the read element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance in the read element and a corresponding change in the sensed current or voltage.
A different and more pronounced magnetoresistance, sometimes called giant magnetoresistance (GMR) or the spin valve effect, has been observed in a variety of magnetic multilayered structures, the essential feature being at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. This GMR effect has been found in a variety of systems, such as Fe/Cr, Co/Cu, or Co/Ru multilayers exhibiting strong antiferromagnetic coupling of the ferromagnetic layers, as well as in essentially uncoupled layered structures in which the magnetization orientation in one of the two ferromagnetic layers is fixed or pinned. The physical origin is the same in all types of structures: the application of an external magnetic field causes a variation in the relative orientation of the magnetizations of neighboring ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes.
A particularly useful application of GMR is a sandwich structure comprising two uncoupled ferromagnetic layers separated by a nonmagnetic metallic layer in which the magnetization of one of the ferromagnetic layers is pinned. The pinning may be achieved by depositing the layer onto an iron-manganese (Fe-Mn) layer to exchange couple the two layers. This results in a spin valve magnetoresistive sensor in which only the unpinned or free ferromagnetic layer is free to rotate in the presence of an external magnetic field. U.S. Pat. No. 5,206,590, filed Dec. 11, 1990, and assigned to IBM, discloses a basic spin valve MR sensor. U.S. Pat. No. 5,159,513, filed Feb. 8, 1991, and also assigned to IBM, discloses a spin valve MR sensor in which at least one of the ferromagnetic layers is of cobalt or a cobalt alloy, and in which the magnetizations of the two ferromagnetic layers are maintained substantially perpendicular to each other at zero externally applied magnetic field by exchange coupling of the pinned ferromagnetic layer to an antiferromagnetic layer.
To meet the requirement for increasing data densities in today's magnetic storage systems, it is required that the magnetic flux sensing layers or read elements in MR sensors (both AMR and spin valve) be fabricated in progressively thinner layers of ferromagnetic material. However, MR sensors utilizing ultrathin magnetic flux sensing layers exhibit degraded magnetoresistance (i.e., deltaR/R) for both AMR and spin valve sensors.